Turbofan gas turbine engines (which may be referred to simply as ‘turbofans’) are typically employed to power aircraft. Turbofans are particularly useful on commercial aircraft where fuel consumption is a primary concern. Typically a turbofan gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven directly off an additional lower pressure turbine in the engine core.
The fan draws air into the engine and the air is discharged through a bypass duct which provides a portion of the engine thrust.
Positioned within the bypass duct, axially rearward of the fan, a series of outlet guide vanes (OGVs) are arranged and positioned between an inner and outer casing that define the bypass duct. A series of acoustic panels line the inner and outer casing. To prevent air recirculating behind the acoustic panels and to provide a smooth surface for air flow through the engine, an infill panel is provided between each of the OGVs.
The infill panel is generally a moulded component that includes acoustic chambers, and is typically bolted to the respective inner or outer casing.
A gap exists between the infill panel and the acoustic panel. The distance of the gap can vary depending on a number of factors, including how tightly sections of the inner and/or outer case are connected (e.g. bolted) together. It is important that the gap between the infill panel and acoustic panel is sealed so as to create a smooth gas washed surface, but also to prevent air flow recirculating behind the acoustic panel and/or infill which could cause damage to the engine.
The conventional method to fill the gap is to use a sealant such as polysulphide.
FIG. 1 shows an arrangement of an infill and forward and rearward acoustic panels forming part of an outer casing assembly of the prior art. The casing assembly includes a fan case 1035 substantially coaxial with and connected to a mount ring 1039. The mount ring 1039 is substantially coaxial with and connected to a rear case 1041. Acoustic panels 1034A line the fan case and acoustic panels 1034B line the rear case 1041. The fan outlet guide vanes (OGVs) 1032 are connected to the mount ring, typically via bolts. An infill panel 1040 is positioned between each OGV to fill a void between the forward acoustic panel 1034A and the rearward acoustic panel 1034B. A gap is formed between the forward acoustic panel 1034A and the infill panel 1040. This gap is filled with polysulphide sealant 1048A.
Use of a sealant is advantageous because an effective seal can be produced independently of the gap size. However, the use of sealant to fill the gap leads to increase weight of the engine, increased lead time due to the cure time for the sealant and increased cost. A further problem is that in-service-support may require the removal of a panel, which is time consuming because of the time taken to completely remove the sealant and then re-inject and cure the sealant.